The present invention relates to a non-aqueous electrolyte secondary battery having an electrode group composed of a thin positive electrode and a thin negative electrode with separator material sandwiched therebetween, and more particularly to its safety.
Conventionally, non-aqueous electrolyte secondary batteries use a chalcogenide, such as an oxide, sulfide or selenide of a transition metal as positive active material. For example, manganese dioxide, molybdenum disulfide, titanium selenide, or metal lithium sheet is used as a negative active material. An organic electrolyte composed of an organic solvent solution of lithium salt is typically used as a non-aqueous electrolyte. Such batteries are typically referred to as lithium secondary batteries, and are aimed at producing batteries of high voltage, large capacity, and high energy density. In such lithium secondary batteries, however, although an intercalation compound having excellent charging and discharging characteristics may have been selected as the positive active material, the charging and discharging characteristics of the negative electrode was not always excellent, and it was difficult to assure a long cycle life. Furthermore, accidents such as fire and rupture due to an internal short circuit were likely to occur, raising serious safety concerns.
The metal lithium in the negative active material in this battery is dissolved as lithium ions in the organic electrolyte due to discharge. When charging, the dissolved lithium ions deposit on the surface of the negative electrode as metal lithium, but all of them do not deposit smoothly as in the initial state. Some of them deposit as dendrites or mossy, active, metallic crystals. Such active metallic crystals react with the organic solvent in the electrolyte, causing their surface to be covered with a passivation film, making them inactive and unable to contribute to discharge. Therefore, the negative electrode capacity drops as the charging and discharging cycles are repeated. Accordingly, when manufacturing the cells, it was necessary to set the negative electrode capacity extremely larger than that of the positive electrode. Besides, active lithium metallic crystals are likely to form an internal short circuit by penetrating through the separator and contacting with the positive electrode. By such an internal short circuit, the cell may suddenly generate hear, causing a cell rupture or accidental fire.
Accordingly, the so-called lithium ion secondary batteries using a material for intercalating and deintercalating lithium ions by charging and discharging as the negative material have been proposed, intensively researched and developed globally, and are now already in a practical stage. The lithium ion secondary battery, as long as it is not overcharged, does not deposit active metallic lithium crystals on the negative electrode surface when charging, and enhancing safety. Its demand is growing rapidly in recent years because it is superior to the conventional lithium secondary battery in high-rate charge and discharge characteristics and life cycle. In the lithium ion secondary battery, lithium is the active material, and thus, the battery may be regarded as a kind of lithium secondary battery. It can be distinguished, however, from the lithium secondary battery that uses conventional metallic lithium as the negative electrode.
As the positive active material of the lithium ion secondary battery, a double oxide of lithium and a transition metal, such as LiCoO2, LiNiO2, LiMnO2, or LiMn2O4 in discharged state, is used. As the negative active material, graphite or other carbon material similar in potential to the metallic lithium as charged is used in most systems, but in other systems of low voltage operation, in part, a double oxide of lithium and transition metal is used in the negative electrode.
When the lithium ion secondary battery is charged and discharged, the positive active material can reversibly repeat deintercalation and intercalation of lithium ions, and the negative active material can reversibly repeat intercalation and deintercalation of lithium ions, so that the cycle life is extremely long. Moreover, because of high voltage and/or large capacity, a battery of high energy density is provided.
However, these lithium ion secondary batteries, like the conventional lithium secondary batteries, employ organic electrolytes of relatively low ionic conductivity. Accordingly, a thin positive electrode and negative electrode are fabricated by thinly forming an active material layer or a mixture layer of active material and conductive agent on a metal foil of current collector. An electrode group is composed by setting the positive electrode and negative electrode oppositely to each other separated by a thin microporous polyolefin resin membrane separator. By increasing the opposing surface areas of the positive electrode and negative electrode, a practical high-rate charge and discharge characteristic is maintained to expand conformity to many applications. For example, the positive electrode and negative electrode, each piece in a thin and long strip form sandwiching a separator therebetween, may be spirally wound or plaited like an accordion, or a plurality of positive electrodes and negative electrodes may be laminated alternately with a separator therebetween to form the electrode group.
In these lithium ion secondary batteries, a separator capable of closing fine pores and thus decreasing the ion conductivity when raised to a specified temperature is used to cut off current. Moreover, an electronic protection circuit in the battery pack is used to control each cell to prevent fatal deterioration due to overcharge and overdischarge. Therefore, when used normally, safety is assured, but in abnormal use, it is hard to guarantee safety. For example, when a battery pack in a fully charged state is crushed by a strong external force, such as being run over by an automobile, or when overcharged due to malfunction of the protection circuit as described above, the separator in the cell may be broken, and the positive and negative electrodes are shorted. Such shorted electrodes generate heat by Joule heat or reaction heat, and when the decomposition temperature of the positive active material is achieved, active oxygen is generated. The active oxygen violently oxidizes the solvent in the organic electrolyte or the other material in the cell, causing a state of thermal runaway. As a result, the cell temperature rises sharply in an instant, possibly leading to cell rupture or accidental fire. The risk of such accident is also present when the charged battery pack is disposed of with common household refuse.
To prevent such accidents, usually, in each cell, a temperature fuse, PTC device, other temperature rise preventive means, and an explosion-proof safety valve are provided, but may not be sufficient to cope with the sudden temperature rise due to a thermal runaway. It was therefore proposed to provide a cell capable of preventing a sudden rise in cell temperature, thus preventing cell rupture and accidental fire as experienced hitherto when the positive electrode and negative electrode are short-circuited, such as when the separator is broken due to the cell being crushed or overcharged. A typical example is disclosed in Japanese Laid-open Patent Application No. Hei8-153542, relating to a laminate electrode assembly (electrode group) comprising a positive electrode and a negative electrode, each comprising an active material layer at least on one side of a metal foil which is a collector, positioned opposite to each other with a separator therebetween. The confronting portions of the metal foils of the collector of the positive electrode and the negative electrode are exposed at least on one side, over at least one turn or one layer or more, with the separator therebetween, in any one of the electrode group outermost portion, innermost portion, or intermediate portion.
In such cell a composition, when the side surface is pressed, the cell is crushed, the separator is torn, and the positive electrode and negative electrode contact each other, the short-circuit current flows selectively between the exposed metal foil portions of the collector of the positive electrode and the negative electrode, which are higher in electronic conductivity than the active material layer, and the positive and negative active materials in a charged state are discharged and consumed in a short time, so that the cell temperature may not be raised to a critical state. Moreover, in order to short-circuit securely between exposed portions of the metal foil of the positive and negative electrode collectors, this same publication also discloses means for selectively tearing the separator between the exposed portions of the positive and negative metal foils by interposing a part made of a rigid or elastic body at least in one of the exposed portions of the positive and negative electrode metal foils.
As a result of close studies of these proposed cell compositions, the present invention proposes a cell composition having an electrode group for selectively short-circuiting in a position of high electronic conductivity between the metal foil of the positive electrode collector and the negative electrode, easily releasing heat in the cell without sacrificing the cell capacity or increasing the number of parts more than necessary. By employing such a cell composition, it is intended to present a non-aqueous secondary battery high in reliability and enhanced in safety, capable of securely preventing accidents such as rupture or fire, even in the event of the abnormality of crushing the cell.
The invention relates to a non-aqueous electrolyte secondary battery comprising an electrode group composed by sandwiching a separator between a thin positive electrode and a thin negative electrode. Each electrode comprises a metal foil, which is a collector, having a thin coating thereon, the coating comprising an active material layer or a mixture layer of active material and conductive agent. The electrode group is configured such that the negative electrode is positioned outwardly relative to the positive electrode, and a portion of exposed metal foil, which is electrically connected to the positive electrode and has no active material layer or no mixture layer of active material and conductive agent thereon, covers the outer side of the negative electrode with a separator therebetween. The outermost side of the exposed metal foil is also covered with a separator. The electrode group so constructed is put in a negative polarity cell container together with non-aqueous electrolyte. Thus, the exposed metal foil connected to the positive electrode collector covers the entire surface of the outer side of the electrode group, having one side facing the negative electrode and the other side facing the inner side wall of the negative polarity cell container. Therefore, if the cell side surface is pushed by a strong external force and the cell is crushed, the separator on one side or both sides of the metal foil of the positive electrode collector positioned outside of the electrode group is first broken, and the metal foil for the positive electrode short-circuits with a least one of the negative electrode and the inner wall of the negative polarity cell container. The positive and negative materials in a charged state are thus discharged and consumed in a short time, and because the short-circuit position is adjacent to the cell container, the heat is released easily, thereby preventing a sudden rise in cell temperature. As a result, cell rupture, fire or other accidents may be prevented, so that the reliability and safety are successfully enhanced without increasing the number of parts or sacrificing the cell capacity more than necessary.